Dimitri Geskus

Hybrid integrated diode lasers based on combining semiconductor optical amplifiers with low-loss Si ₃ N ₄ -based feedback circuits enable great laser performance for advanced photonic circuits. In particular, using high-Q Si ₃ N ₄ ring resonators for frequency-selective feedback provides wide spectral coverage, mode-hop free tuning, and high frequency stability on short timescales, showing as ultra-narrow intrinsic linewidths. However, many applications also require long-term stability, which can be provided by locking the laser frequency to a suitable reference. We present the stabilization of a hybrid-integrated laser, which is widely tunable around the central wavelength of 1550 nm, to a fiber-based optical frequency discriminator (OFD) and to an acetylene absorption line. By locking the laser to the OFD, the laser's fractional frequency stability is improved down to 1.5 · 10 ⁻¹² over an averaging time of 0.5 ms. For absolute stability over longer times of several days, we successfully lock the laser frequency to an acetylene absorption line. This limits the frequency deviations of the laser to a range of less than 12 MHz over 5 days.

Wavelength stabilization of external cavity lasers is a of key importance to exploit their sub-kHz intrinsic linewidth. In this work we demonstrate < 20 dB optical phase noise reduction at acoustic frequencies using a simple off-the-shelf electronic feedback loop. The novelty here is that we exploit an on-chip optical frequency discriminator (OFD) in Si3N4 (TriPleX), based on an aMZI with a path length difference of 1.4 m, having less than 10 dB loss. The used setup has a bandwidth of approximately 1 MHz, allowing for wavelength modulation depth in the order of tens of MHz.

Optical frequency combs based on diode lasers are becoming instrumental for numerous applications based on coherent light, such as sensing, metrology, or integrated microwave photonics. However, standard mode-locked diode lasers show limits in coherence due to intrinsic loss of semiconductor amplifiers and short laser cavity lengths. Extending the cavity with long and low-loss dielectric waveguide feedback circuits, specifically using advanced Si 3 N 4 waveguides [1] , has enabled single-frequency lasers with record-low intrinsic linewidths [2] . This approach was adapted also for demonstrating multi-frequency lasers [3] , but only recent results have proven comb generation [4] using heterogeneous integration [5] . A highly stable repetition rate was achieved and a high comb density (sub-GHz line spacing). The spectral coverage, however, remained low, about 2.5 nm of 3dB bandwidth at 1582 nm, which may be addressed to increased coupling losses and limited pump currents associated with the heterogeneous integration.

The spectral response of a distributed-feedback resonator with a thermal chirp is investigated. An Al₂O₃ channel waveguide with a surface Bragg grating inscribed into its SiO₂ top cladding is studied. A linear temperature gradient along the resonator leads to a corresponding variation of the grating period. We characterize its spectral response with respect to wavelength and linewidth changes of the resonance peak. Simulated results show good agreement with the experimental data, indicating that the resonance wavelength is determined by the total accumulated phase shift. The calculated grating reflectivities at the resonance wavelength largely explain the observed changes of the resonance linewidth. This agreement demonstrates that the linewidth increase is caused by the increase of resonator outcoupling losses.

Here we report 10 laser emission lines in the attractive deep blue to cyan spectral region from an intracavity frequency doubled Raman laser. The fundamental laser field that drives the Raman laser is based on the three-level transition of Nd:YLF. A maximum extracted quasi-continuous wave (qcw) output power of 0.94 W is achieved in the deep blue to cyan spectral regime.

An intracavity Raman laser based on laser transition in Nd:YLF is reported. Quasi-continuous wave Stokes output power was observed at 990 and 976 nm, and up to 0.88 W of peak output power was obtained at 990 nm.

Using a commercial, diode-side pumped Nd:YAG rod laser module, we obtain more than 53% extraction efficiency in fundamental mode with respect to multimode operation in a fully polarized beam.

Channel waveguide lasers operating at 981 nm are demonstrated in KY_1−x−yGd_xLu_y(WO₄)₂:Yb³⁺ waveguides grown by liquid phase epitaxy onto undoped KY(WO₄)₂ substrates and microstructured by Ar⁺ beam etching. Under pumping at 934 nm of samples with different waveguide geometry and outcoupling degree, a record-high slope efficiency of 76% versus absorbed pump power and a record-high output power of 650 mW for rare-earth-ion-doped microstructured channel waveguide lasers is achieved. The laser performance is compared to that of the same devices when pumping at 981 nm and lasing near 1025 nm.

Modal gain per unit length versus launched pump power is predicted and measured in a 47.5 at.% Yb³⁺-doped potassium double tungstate channel waveguide. The highest measured gain exceeds values previously reported for rare-earth-ion-doped materials by two orders of magnitude.